1. Field of the Disclosure
This disclosure relates to the acquisition of seismic data using seismic spreads.
2. Background of the Art
Seismic surveys are conducted to map subsurface structures, which maps are used to locate and develop oil and gas reservoirs. On land, seismic surveys are conducted by deploying a large array of seismic sensors over selected geographical regions. Typically, these arrays can cover several square kilometers (for example 100 square kilometers) of a geographical area and may include in excess of 30,000 seismic sensors (also referred to as receivers) placed in the ground and arranged in the form of a grid. The receivers are typically geophones and/or accelerometers for land operations. Three-axis accelerometers are often used as receivers.
An energy source, such as an explosive charge (buried dynamite, for example) or a mobile vibratory source is typically used at selected locations in the array to generate acoustic waves or signals (also referred to as acoustic energy) that propagate through the subsurface structures of the earth. The generated acoustic waves reflect at subsurface formation discontinuities, such as boundaries associated with layers of different rock types, salt domes and oil and gas reservoirs. These reflections are sensed at the surface by the seismic sensors in the array. Sensors are typically grouped in small numbers and each group is connected to a separate data acquisition unit (also referred to as a recording unit, or a field service unit). Each data acquisition unit receives the signals from its associated sensors, samples the signals, digitizes the samples, stores the digitized samples, arranges the digitized samples into packets and transmits such packets to a central control unit (also referred to as a central recording unit), either directly or via one or more intermediate units and/or repeaters.
The recorders may transmit the packets via cables or wirelessly to the central control unit, which may be on a mobile unit, such as a truck or at another remote location. The central control unit typically processes the data (at least partially) received from the data acquisition units, stores the processed data for later processing and may send the processed data to another remote unit for further processing of the data. A two or three-dimensional map (also referred to as a seismic image) of the subsurface structures is generated by processing of the data received from the central control unit.
Offshore seismic data acquisition systems typically utilize a compressed air source, such as an air-gun, as the seismic energy source, which is activated at selected locations a few meters (often 5-6 meters) below the water surface while being towed by a vessel. The receivers are deployed either in streamer cables that are towed by the vessel carrying the source or are deployed at the ocean bottom in the ocean-bottom cables. Hydrophones are typically used as the receivers for offshore applications.
In a seismic spread, each recorder transmits a large number of packets. Typically, each packet may contain an “epilog,” a payload and a “prolog” that includes a large number samples or words (for example, about 500), each sample having a prescribed number of bits (for example, twelve bits or twenty-four bits, etc). Often, the useful portion of the word includes less than the total number of available bits. Therefore, some or many samples in a packet may occupy bit spaces that contain only leading sign bits. The leading sign bits are simply “ones” or “zeros.” Accordingly, there is a need for an improved method and apparatus for preparing, storing and transmitting packets.
Also, it is known that the recorder units in seismic spreads experience coherent noise. Coherent noise is periodic in nature. It is typically undesirable seismic energy that shows a consistent phase from trace to trace, such as ground roll and multiples. Coherent noise can occur due to several different factors, such as: the presence of a common mode induction at the receiver input due to data transmission; and electronic switching in the intrinsic circuits coupled into the receiver by various methods, such as telemetric transmitters radiating energy, limited power supply noise rejection, common circuit elements such as power supplies or ground planes, and high energy computation bursts such as those present during data transfer or intrinsic math functions. Therefore, there is a need for a method and an apparatus that may reduce the coherent noise in seismic data acquisition systems.